The present invention relates to capacitors, particularly to high performance capacitors, and more particularly to high performance, high energy-density capacitors fabricated from nano-structure multilayer materials, and method for fabricating same.
Many pulse power and industrial applications are limited by capacitor performance. Capacitor requirements for military and scientific pulse power applications are particularly acute. The rapid discharge of a significant amount of electrical energy is used to create a variety of physical phenomena. To achieve this, high voltage, high energy density capacitor banks with good circuit performance are required. Capacitors for pulse power applications must have low loss, low inductance and be thermally and mechanically robust. The pulse power applications include electrothermal and electromagnetic propulsion, x-ray generation, and electromagnetic effects. Industrial applications include high precision instrumentation, medical instrumentation and systems, space and remote applications, and high reliability electronics.
Military and space applications have uniquely difficult survivability and reliability requirements for harsh environments. Capacitor banks need to survive and function through mechanical and thermal shock without significant mechanical degradation or chemical aging. In some cases, capacitors need to survive high radiation environments. Military applications in the field must be maintainable while remote applications in space depend on highly reliable capacitors for electrical energy storage.
New applications demand better capacitor performance than currently available. Capacitors and capacitor bank requirements include high energy density, specific energy, bank charging voltage, and efficiency. Also, about 1Hz repetition rate and long shot file (&gt;1000) are needed for many applications. Current state-of-the-art capacitors can achieve an energy density &lt;1 MJ/m.sup.3 and a specific energy of &lt;0.6 KJ/Kg delivered, while requirements for a current electrothermal propulsion system, for example require: energy density of .gtoreq.15 MJ/m.sup.3, specific energy of .gtoreq.10 KJ/Kg, bank charge voltage of 5-20 Kv, loss factor of &lt;0.02. Such requirements are not achievable with existing capacitor technology. While incremental improvements are anticipated from existing capacitor technologies, significant advances are needed to meet these requirements.
Current capacitor technologies suffer from defects and in homogeneity introduced in the dielectric material and in capacitor manufacturing which contributes to voltage breakdown. Important voltage breakdown mechanisms include electrical or avalanche breakdown, electrochemical breakdown, and thermal breakdown as dielectric loss increases under stress and with aging.
Rolled paper-conductor or polymer-conductor capacitors are characterized by high breakdown voltage but lower dielectric constant. They suffer from material and manufacturing defects, and can fail under the mechanical and thermal shock expected of pulse power applications in military environments. They experience chemical degradation with aging, temperature and shot life. They are bulky to package and aren't expected to achieve the performance required for the pulse power applications currently being developed.
Capacitors with ferroelectric ceramic dielectrics, such as BaTiO.sub.3, typically have a much higher dielectric constant than polymer- or paper-conductor capacitors. However, large geometry ceramic capacitors still have lower breakdown voltage and often high dielectric loss. Performance of the current ceramic capacitors is limited by ceramic powder quality, capacitor design, and the manufacturing process.
It is thus seen that due to the performance of the currently known capacitors, many pulse power and industrial applications are limited. While incremental improvements are anticipated from existing capacitor technologies, significant advances are needed in energy density, specific energy, life time, etc. to enable the desired capacitor applications for both the military and for American economic competitiveness.
It has been recognized that capacitor structures fabricated using nano-engineered multilayer technology will give the ability to engineer high performance, high energy-density capacitors. Nano-engineered multilayers are characterized by a near atomic scale and thus, uniquely large interfacial area to volume ratio. Thus, using this technology, capacitor properties can be optimized by material selection, and design of the synthesis process and materials processing.
Multilayer materials are widely known in the materials community for scientific study and physics applications. Their use has been demonstrated widely. For example, see U.S. Pat. Nos. 4,673,623 issued Jun. 16, 1987 to D. S. Gardner et al.; 4,870,648 issued Sep. 26, 1989 to N. M. Ceglio et al., and 4,915,463 issued Apr. 10, 1990 to Troy W. Barbee. In addition, multilayer thin films have been deposited using a magnetron sputter deposition process described and claimed in copending U.S. application Ser. No. 07/666,971 filed Mar. 11, 1991, now U.S. Pat. No. 5,203,977, and assigned to the same assignee.
Various sputtering techniques have been developed, and such are exemplified by U.S. Pat. Nos. 4,049,533; 4,183,797; 4,392,939; 4,417,968; 4,421,628; 4,915,805; 5,021,139; and 5,022,978.
Sputtering is a vacuum coating process where an electrically isolated cathode is mounted in a chamber that can be evacuated and partially filled with an inert gas. If the cathode material is an electrical conductor, a direct-current high-voltage power supply is used to apply the high voltage potential. If the cathode is an electrical insulator, the polarity of the electrodes must be reversed at very high frequencies to prevent the formation of a positive charge on the cathode that would stop the ion bombardment process. Since the electrode polarity is reversed at a radio frequency of 13.56 MHz, this process is referred to as RF-sputtering.
Magnetron sputtering is a more effective form than diode sputtering that uses a magnetic field to trap electrons in a region near the target surface creating a higher probability of ionizing a gas atom. The high density of ions created near the target surface causes material to be removed many times faster than in diode sputtering. The magnetron effect is created by an array of permanent magnets included within the cathode assembly that produce a magnetic field normal to the electric field.
Ion bombardment not only causes atoms of the target material to be ejected, but also imparts considerable thermal energy to the target. Consequently, any target attachment scheme must provide for good physical contact to the cathode assembly to allow adequate thermal transfer of the target's heat to the cooling media or away from the sputtering source. This is particularly true in the case of magnetron sputtering where very large ion currents are produced causing a very intense and localized heating of the target.
Various means have been used in the past for holding sputter targets in place within the sputter sources. Commercially available sputter coating target cathodes today are either bonded directly to the cathode assembly or secured using various mechanical means. The method used to attach the sputter target to the cathode assembly will also greatly affect the size and overall design of the magnetron sputtering source, the amount of down time when changing targets, and the overall performance of the source. By also eliminating the need for a cooling fluid, the miniaturization of the magnetron sputtering apparatus becomes feasible, and the cost of building the sputtering apparatus becomes more economical.
A co-pending U.S. application Ser. No. 07/962,657, filed Oct. 19, 1992, now U.S. Pat. No. 5,286,361, issued Feb. 15, 1994, assigned to the assignee of this invention, discloses and claims a method and assembly for attaching sputtering targets to cathode assemblies using a magnetically permeable material imbedded in the base portion of the sputter target. Target attachment to the cathode is achieved by virtue of the permanent magnets and/or pole pieces that create magnetic flux lines adjacent to the backing plate, which strongly attract the magnetically permeable material in the target assembly. Also, copending U.S. application Ser. No. 08/005,122 filed Jan. 15, 1993, now U.S. Pat. No. 5,333,726, issued Aug. 2, 1994, assigned to the same assignee, describes and claims an improved magnetron sputtering source that is compact, versatile, and less costly to manufacture and operate than prior art sputtering sources.
While other sputtering techniques may be used, magnetron sputtering sources being developed or now in use are deemed to provide more efficient operation and are computer controlled to enable the deposition of this (submicron) layers.
From the prior art it is recognized that all magnetron sputtering sources have four basic design features. First, they all contain a directly cooled and electrically isolated cathode assembly that supports the sputter target and contains the permanent magnet assembly.
Second, a ceramic or polymer insulator electrically isolates the cathode assembly from ground. The insulator structure also establishes the vacuum compatibility of the source by way of ceramic-to-metal braze joints or elastomer O-ring seals.
Third, only a portion of the power in the sputtering process is consumed by ejecting near surface atoms by ion bombardment. Most of the power contributes to thermal heating of the sputter target and the cathode assembly. The excess heat generated is removed by circulating water or another cooling media in the cathode assembly.
Fourth, the sputtering process should be confined to the target surface by using sputter shields that prevent all other exposed surfaces of the cathode assembly and/or target attachment hardware from also being sputtered. This is a necessary requirement if contamination is to be avoided.
Sputter deposition of refractory oxide compound films of high quality from compound targets is difficult as a result of the low mechanical integrity of such targets, their low thermal conductivities, inherently low deposition rates, poor stoichiometry control, and the often substantial contamination incurred during the sputter target preparation. These difficulties may be diminished by reactive sputtering in which the target, consisting of the metal component, is sputtered using a gas mixture consisting of an inert gas (typically argon) and a reactive component, one element of which is active in the formation of the desired compound. Despite present drawbacks associated with a difficulty in closely controlling film stoichiometry, reactive sputtering yields much higher quality films at deposition rates somewhat higher than those attained using compound targets.
The purpose of the program under which this invention arose was to develop a reactive deposition synthesis process with which refractory oxides of controllable structure and composition could be formed in a reproducible and routine manner at substantially increased rates. The studies on the reactive deposition of the oxides of titanium and zirconium (TiO.sub.x, ZrO.sub.x) have been reported. Films of TiO.sub.x and ZrO.sub.x (O&lt;x&lt;2) were synthesized at high rates onto substrates held at room temperature. A systematic investigation of the structure-composition-synthesis process variables was conducted for these systems, and it was clearly demonstrated that films of controllable stoichiometry, varying over the range Ti, ZrO.sub.0.1, to Ti, ZrO.sub.2, can be reproducibly synthesized at rates (5 .ANG./sec) equal to or greater than those typically encountered in industry. In addition, the substrate temperature during synthesis was close to the ambient (T.sub.s =65.degree. C.). The low temperature nature of this process is a unique feature of high industrial potential, as it provides a method for forming refractory oxide materials (Ti, ZrO.sub.x films) that does not require elevated temperature cycling which could degrade any other existing materials performance.
The experimental approach of the above-referenced studies was based on the concept that isolation of the reaction between the sputtered vapor species (Ti, Zr in this case) and the reactive gas (O.sub.2) to the substrate deposition surface would increase the oxide deposition rate and increase reproducibility as the oxidation reaction characteristics at that position are not plasma affected. High deposition rates are attainable as they are determined by the deposition rates of the metal component which are often at least an order of magnitude higher than their oxide. Control of structure and stoichiometry would be attainable through control of the relative rates of incidence of the two reactive components onto the deposition surface and the fact that the local environment at the deposition. Also, if such control was realized, it would facilitate both modeling of the reactive deposition process and enhance experimental reproducibility in both a given laboratory and between laboratories.
The experimental arrangement used was designed to isolate the sputter-source plate from the oxygen gas, maintaining it in an inert argon gas atmosphere while introducing an oxygen pressure at the substrate. An apparatus designed so that a range of oxygen pressures at the substrate could be maintained and that the distribution of oxygen at the deposition surface would be uniform was developed. A gas ring sources for oxygen, was fabricated with the gas inlet in consisting of slits 0.01 in. wide on the inner diameter of a two-piece annular structures. The substrate was mounted in the ring with the oxygen directed down toward and across the substrate surface. Argon was introduced into the ground shield of the sputter source resulting in an argon stream flowing across the sputter target surface. Acceptable film uniformity both in thickness and composition was attained with this appratus. The general oxygen pressure varied from 0 to 0.6 mtorr (0.08 Pa) in the total system, considerably lower than the oxygen pressure at the substrate, which has not been measured in these experiments.
The experimental results demonstrate that the synthesis of M-oxygen films under controlled conditions, designed so that reaction between M and oxygen occurs at the deposition surface, allows formation of films of reproducible stoichiometry and high uniformity onto room temperature substrates. In our experiments, a dynamic moving surface, in which both silicon and oxygen are supplied to a moving surface (i.e., where a film is being grown) is studied.
The relationship between coverage and exposure is similar to the relationship between composition and relative rates of incidence. A more direct comparison can be made if our data are displayed in terms of the oxygen concentration given as x in MO.sub.x, and a dynamic exposure L.sub.d. The correspondence between coverage and x in MO.sub.x, is clear if we assume that the absorbed species is 0, and that unit coverage is attained when there is one x oxygen atoms absorbed for each M surface atom. Dynamic exposure L.sub.d is defined by the time necessary to deposit a monolayer of M atoms and is used in the calculation of oxygen exposure at pressure P. The number of atoms on an exposed metal surface is calculated to be .about.3.96.times.10.sup.15 /cm.sup.2. The rate of incidence of M atoms is given as the film thickness deposited per unit time divided by the atomic volume of M.
Thus, a dynamic exposure L.sub.d for oxygen can be defined as EQU L.sub.d =P.sub.o .tau.
where P.sub.o, is given in torr and .tau. in seconds. During a steady-state synthesis experiment, static exposure and dynamic exposure are analytically equivalent at small time steps.
A series of experiments, in which the reaction between oxygen and M during reactive sputtering is forced to occur at the deposition surface, has been performed and analyzed. A suitable technique was developed for isolating the magnetron sputter source from the ambient oxygen and MO.sub.x, films over the complete range of stoichiometry, O&lt;x&lt;2, prepared. These films were of excellent uniformity in both thickness and index of refraction.
A sticking coefficient for oxygen of 1.5 to 4.times.10-3 was inferred. The sticking coefficient was found to be independent of coverage (i.e., surface stoichiometry). An appropriate model was deduced to be physisorbed oxygen in a mobile precursor state prior to chemisorption. Such a model yields a consistent explanation of the temperature dependence of the stoichiometry of reactively sputtered oxides in general and MO.sub.x, in particular. These results demonstrate that fundamental information concerning the interaction of oxygen and various elements is accessible through systematic reactive deposition expeirments in which the reaction is localized to the deposition surface.
Nano-structure or nano-phase multilayer materials are dense, low contamination solids synthesized using atom by atom processes. They are characterized by a high concentration of material interfaces. The most notable of such materials as semiconductor superlattices fabricated using molecular beam epitaxy (MBE). However, multilayers may be synthesized using elements from all parts of the Periodic Table Using MBE, evaporation, sputtering and electrochemical deposition technologies. At this time, multilayer structures have been fabricated by physical vapor deposition from at least 75 of the 92 naturally occurring elements in elemental form, as alloys or as compounds. The structure of multilayer materials is determined in synthesis by control of the thicknesses of the individual layers during deposition. These thicknesses vary from one monolayer (0.2 nm) to thousands of monolayers (&gt;1000 nm).
Until recently, the macroscopic thickness of nano-structure multilayer materials has been generally limited to less than a few microns, and more typically to 0.5 .mu.m or less. Recently, processes for magnetron sputter deposition of thick macroscopic nano-structure multilayer materials have been developed and used to fabricate free standing high quality structures up to 300 .mu.m thick containing up to 50,000 individual layers. Existing research synthesis systems produce samples having periods uniform to 2% of the individual layer thickness and areas of .about.400 cm.sup.2. These macroscopic nano-structure multilayer samples enable use of standard diagnostic techniques for material property characterization and open a path to develop devices with performance that approaches theoretical limits.
The through film and lateral perfection of these macroscopic multilayer materials have been determined using surface roughness measurements, cross-section transmission electron microscopy (TEM) and standard x-ray diffraction analysis. The surfaces of macroscopic multilayers (t&gt;20 .mu.m) have demonstrated surface perfection essentially equal to the substrate roughness: multilayers deposited on super polished substrates with roughness of .about.0.02 nm RMS and 0.14 nm peak to valley (PV) had roughness of .about.0.04 nm and 0.29 nm PV. Cross-section TEM shows that the multilayer structure .about.17 .mu.m from a substrate is identical to that 1 .mu.m from that same surface and that this uniformity extended laterally over several microns. X-ray analysis demonstrates that the multilayer period of a 5 nm period 25 .mu.m thick free standing structure varied by less than 1% top to bottom through 10,000 individual layers and is constant over 10 cm on single substrates. The perfection shown by these characterization results is unique in that it is atomic in scale but extends over macroscopic distances. These materials exhibit exceptional application specific performance as a result of their nano-structures and atomic distributions. Structural flaws that characteristically limit performance are controlled so that the full potential of the nano-structure multilayer materials is achievable.
The limited performance of the state-of-the-art capacitors is overcome by the present invention, which, as pointed out above, is based on the recognition that capacitor structures could be fabricated using existing nano-engineered multilayer technology. The advantages to fabrication of high energy-density capacitors by multilayer synthesis technologies include: 1) a wide range of materials and thicknesses may be used, 2) the structures are thermally and mechanically robust, 3) the layers are very smooth, and 4) the materials are pure, homogeneous, and defect-free. Thus, capacitors made in accordance with the present invention will be able to satisfy the abovereferenced needs for pulse power and industrial applications.